1 Answer
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The counter goes up when you purchase one, just like Buildings, so it's a limit on how many you can purchase. From the wiki article on the Vernal Fountain it sounds like the game lags if you have too many of these items (they're animated) so they limit you to prevent you from crashing the game.

I own one of these and I can confirm it doesn't give a boost to Bloom dragons at all and appears perfectly decorative. None of the other decorations have in-game benefits either (other than raising the meaningless Visitors counter) so this seems normal.